The invention relates to a fast neutron nuclear reactor equipped with at least one residual power removal device.
It is known that in a fast neutron reactor, the reactor core is immersed in a predetermined volume of liquid metal (generally sodium) contained in a vertically axed vessel, sealed in its upper part by a horizontal sealing slab. In normal operation, the power given off by the fision reaction in the reactor core is absorbed by the circulation of the liquid metal in a primary circuit incorporating the pre-vacuum pumps and the intermediate exchangers respectively ensuring in operation the continuous circulation and cooling of the liquid metal. In integrated reactors the complete primary circuit is located in the reactor vessel, whereas it passes out of the vessel in the case of a loop-type reactor. In general, the heat extracted from the reactor core by the liquid metal of the primary circuit is transferred to a liquid metal (generally sodium) circulating in a secondary circuit comprising steam generators which, in turn, transfer the heat to a water/steam circuit operating the turbines of an electricity generating plant.
It is obvious that in the case of an operational accident leading to the stoppage of the pre-vacuum pumps, the core in which the fission reactor is immediately stopped due to the dropping of the scram rods, still gives off a large amount of residual calorific power, which should be reliably and effectively eliminated in order to prevent local melting of the core.
For this purpose, it is conventional practice to provide loops or circuits for cooling the reactor when shut down and they comprise heat exchangers directly immersed into the liquid metal contained in the vessel and pumps ensuring the circulation of the liquid metal (generally sodium) circulating in said loops or circuits in order to remove the residual power from the core by means of liquid metal/air exchangers.
Although such shutdown reactor cooling loops or circuits have quite satisfactory operating characteristics, the numerous components (exchangers, expansion vessel, electromagnetic pumps, purification system, sodium storage tanks) located on these loops, as well as the level difference (approximately 18 m) between the exchanger immersed in the sodium and the air exchanger (level difference which is added to the height of the chimney or flue ensuring the cooling of the sodium-air exchanger) make their construction relatively complex and therefore costly. Moreover, the operation of these loops necessitates an external mechanical energy supply at the pumps ensuring the circulation of the liquid metal. From the safety standpoint, this feature is obviously not satisfactory, because it renders these loops ineffective in the case of failure of the electric power supply circuit. Finally, the quantity of heat removed by the shutdown reactor cooling loops must be adjusted as a function of the temperature of the liquid metal in the vessel.